The present invention relates to partially branched polymers having a number-average molecular weight Mn in the range from 500 to 20 000 daltons.
Low molecular weight polymers, i.e., polymers having number-average molecular weights of 20 000 daltons maximum, are of interest for a large number of applications, examples including the preparation of coating compositions in the paints field, the preparation of paper coating compositions, printing inks, adhesives, and floor polishes. In the aforementioned applications they are frequently used as cobinders or auxiliaries. Furthermore, low molecular weight polymers are employed as industrial waxes and also as additives for motor fuels and oils, whether as detergents or as thickeners. An important advantage of low molecular weight polymers over their higher molecular weight homologs is the lower viscosity of these polymers and their solutions, which generally means that less solvent is expended in their processing.
An important class of low molecular weight polymers are those known as functionalized polymers, containing two or more reactive functional groups. Reactive functional groups are those functional groups which are capable of forming bonds with other functional groups at room temperature or on heating. They are therefore used as building blocks in the preparation of polymers of higher molecular weight. For example, low molecular weight polymers functionalized with hydroxyl groups are used as building blocks in the preparation of polyurethanes and polyesters. Furthermore, functionalized polymers are used as a reactive component in thermosettable coating compositions and also in the reaction injection molding (RAM) process.
U.S. Pat. No. 5,534,598 discloses low molecular weight acrylic polymers which are derived from allyl alcohols or propoxylated allyl alcohols and which therefore include OH functional groups. U.S. Pat. No. 4,117,235 and DE 42 03 277 disclose processes for preparing low molecular weight polymers which may contain reactive functional groups.
The prior art low molecular weight polymers based on ethylenically unsaturated monomers generally have a linear structure, having been prepared using exclusively monoethylenically unsaturated monomers. Polyethylenically unsaturated monomers have not been used to date to prepare such low molecular weight polymers on account of the fact that they lead to a rapid buildup in molecular weight and, in extreme cases, to the development of a high molecular weight, three-dimensional network.
For example, DE 36 30 187 describes copolymers based on from 70 to 99.5% by weight of diethylene glycol monomethacrylate and from 0.5 to 30% by weight of esters and/or amides of acrylic acid or of methacrylic acid, containing from 0.1 to 2% by weight of a crosslinking bifunctional monomer in copolymerized form. The polymers obtained by polymerization at low temperatures, e.g. 60xc2x0 C., are high molecular weight networks, which combined with the hydrophilic monomer bases results in the polymers having a high level of swellability by water.
It is an object of the present invention to provide new polymers.
We have found that this object is achieved by purposively copolymerizing ethylenically unsaturated monomers including up to 20% by weight of monomers containing at least two nonconjugated ethylenically unsaturated double bonds to prepare low molecular weight polymers at least some of whose chains contain one or more branching sites (partially branched polymers).
The present invention accordingly provides partially branched polymers having a number-average molecular weight Mn in the range from 500 to 20 000 daltons and synthesized from ethylenically unsaturated monomers including
i) from 80 to 99.9% by weight and preferably from 90 to 99% by weight of monoethylenically unsaturated monomers A, and
ii) from 0.1 to 20% by weight and preferably from 1 to 10% by weight of monomers B containing at least two nonconjugated ethylenically unsaturated double bonds,
the weight fraction of the monomers A and B being based on the total amount of the ethylenically unsaturated monomers that constitute the polymer.
The present invention additionally provides a process for preparing such polymers, which comprises copolymerizing the aforementioned monomers A and B in the specified amounts at temperatures above 150xc2x0 C., preferably 160xc2x0 C., and in particular above 170xc2x0 C.
The molecular weights Mn indicated here and below are number-average molecular weights as may be determined, for example, by exploiting the colligative properties of the polymers, e.g., by vapor pressure or membrane osmosis, by ebullioscopy, by cryoscopy or by gel permeation chromatography. The Mn figures stated here were determined by means of gel permeation chromatography (GPC) using polystyrene standards. The process used is described, for example, in DIN 55672-1.
The polymers of the invention preferably have a number-average molecular weight Mn in the range from 700 to 10 000 daltons and in particular in the range from 1 000 to 6 000 daltons.
The monoethylenically unsaturated monomers A used to prepare the polymers of the invention include compounds containing an ethylenically unsaturated polymerizable double bond. Preferably at least 60% by weight and in particular at least 80% by weight of the monomers A have a polymerizable double bond in the form of an acrylic or methacrylic group.
In one preferred embodiment the polymers of the invention contain a reactive group. Reactive groups are functional groups suitable for crosslinking reactions, whether at room temperature or at elevated temperature. They include carboxyl, ketone, aldehyde, isocyanate, amino and, in particular, hydroxyl and epoxy groups. Monomers containing reactive functional groups are therefore referred to below as monomers A1 and those without reactive functional groups are referred to as monomers A2. In this preferred embodiment of the present invention, the proportion of the monomers A1 among the overall amount of the monomers is generally from 10 to 80% by weight and preferably from 20 to 70% by weight.
Preferred monomers A1 are esters of acrylic acid and of meth-acrylic acid that contain OH groups or oxirane groups. Examples of OH-bearing esters of acrylic acid and/or of methacrylic acid are the C1-C8 hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl acrylate, 2- or 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 6-hydroxyhexyl acrylate, and the corresponding methacrylates. Examples of esters of acrylic acid and of methacrylic acid that carry oxirane groups are glycidyl acrylate, glycidyl methacrylate, and 2,3-epoxycyclohexyl or 3,4-epoxycyclohexyl acrylate or methacrylate. Examples of carboxyl monomers are acrylic acid and methacrylic acid and also, furthermore, fumaric acid, maleic acid, and the monoesters of these acids with C1-C8 alkanols. Examples of amino monomers A1 are 2-aminoethyl acrylamide, 2-aminoethyl methacrylamide, 2-amino-ethyl acrylate and methacrylate, and the corresponding mono- and di-C1-C4 alkylamino compounds.
Examples of the preferred monomers A2 are C1-C20 alkyl acrylates and methacrylates and C5-C10 cycloalkyl acrylates and methacrylates, it being possible for the aforementioned compounds to be halogenated in the alkyl and/or cycloalkyl moiety or to contain one or two nonadjacent oxygen atoms and/or imino groups instead of a CH2 group.
Examples of such monomers include the following: methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylbutyl acrylate, 2-ethylhexyl acrylate, 2-propylheptyl acrylate, lauryl acrylate, stearyl acrylate, cyclopentyl acrylate, cyclohexyl acrylate, 3-methoxybutyl acrylate, 2-methoxybutyl acrylate, 2-ethoxyethyl acrylate, tetrahydrofuryl acrylate, and the corresponding esters of methacrylic acid. Further suitable monomers A2 containing acrylic or methacrylic groups are phenyl acrylate, benzyl acrylate, phenethyl acrylate, 2-phenoxyethyl acrylate, acrylamide, methacrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-phenylacrylamide, oxazolidinylethyl acrylate, methyl xcex1-chloroacrylate, methyl 2-cyano-acrylate, 2-nitro-2-methylpropyl acrylate, and the corresponding methacrylates and methacrylamides.
Further suitable monomers A2 include vinylaromatic monomers such as styrene, xcex1-methylstyrene, tert-butylstyrene, vinyltoluenes, and chlorostyrenes. Further monomers A2, moreover, are the vinyl esters and allyl esters of aliphatic carboxylic acids having from 1 to 20 carbon atoms, such as vinyl acetate, vinyl propionate, vinyl valerate, vinyl versatate (vinyl esters of Versatic(copyright) acids), vinyl laurate and vinyl stearate, and also vinyl halides such as vinyl chloride and vinylidene chloride, vinyl ethers of aliphatic C1-C20 alcohols such as vinyl isobutyl ether, vinyl dodecyl ether and vinyl octadecyl ether, and also heterocyclic vinyl compounds such as vinylpyrrolidone, vinylcaprolactam, vinylimidazole, vinylpyridines, and the like.
The proportion of the monomers A2 among the total amount of the monomers that constitute the polymer is preferably at least 20% by weight and in particular at least 30% by weight. In general it will not exceed 99.9% by weight, preferably 99% by weight. Where monomers A1 are present it will in particular not exceed 90% by weight and with particular preference will not exceed 80% by weight.
Among the monomers B, which contain at least two nonconjugated double bonds, preference is given to those compounds in which at least one, preferably at least two, and in particular all of the double bonds are present in the form of acrylic and/or methacrylic acid groups. In particular, at least 80% by weight of the monomers B, and preferably all monomers B, are selected from those monomers B in which the polymerizable double bonds are present in the form of acrylate and/or methacrylate groups. Examples of such monomers B are derived from diols or polyols that have been at least doubly esterified with acrylic acid and/or methacrylic acid. They include 1,2-ethanediol diacrylate, 1,3-propanediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene glycol diacrylate, and tripropylene glycol diacrylate, trimethylolpropane di- and triacrylate, pentaerythritol tri- and tetraacrylate, and also the corresponding methacrylate compounds.
Further suitable monomers B include the vinyl, allyl and methallyl esters of ethylenically unsaturated carboxylic acids, especially of acrylic acid and of methacrylic acid, such as allyl (meth)acrylate, methallyl (meth)acrylate, cyclohexenyl acrylate and methacrylate, and also dihydrodicyclopentadienyl acrylate and methacrylate, and also the vinyl, allyl and methallyl ethers of polyols, such as butanediol divinyl ether, butanediol diallyl ether, and trimethylolpropane triallyl ether. Further monomers B that may be mentioned by way of example include diallyl phthalate, N,N-divinyl- and N,N-diallylimidazolinone, N,N-divinylurea and N,N-diallylurea, and divinylbenzene.
The polymers of the invention are prepared by free-radical copolymerization of the monomers A and B in the presence of a polymerization initiator. In accordance with the invention the polymerization may be conducted in bulk or in solutions.
The polymerization temperature is generally at least 170xc2x0 C. In general the polymerization temperature will not exceed 300xc2x0 C., preferably 250xc2x0 C., and in particular 220xc2x0 C.
Suitable polymerization initiators include in principle all compounds which are able to trigger a free-radical polymerization of ethylenically unsaturated monomers. The polymerization initiators are preferably selected from organic hydroperoxides, azo esters and organic peroxides, the last-mentioned being particularly preferred.
Examples of organic hydroperoxides are cumene hydroperoxide, tert-amyl hydroperoxide, tert-butyl hydroperoxide, and diisopropylbenzyl monohydroperoxide. An example of azo esters is 2,2xe2x80x2-azodi(2-acetoxy)propane. Preferred organic peroxides are those in which the peroxy group is attached to tertiary carbon atoms, such as dicumyl peroxide, tert-butyl cumyl peroxide, di-tert-butyl peroxide, di-tert-amyl peroxide, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
In the process of the invention the initiator is employed preferably in an amount of from 1 to 6% by weight and in particular from 2 to 4% by weight, based on the monomers to be polymerized. It is also possible to employ larger amounts of polymerization initiator, for example, up to 8% by weight, based on the monomers to be polymerized. In that case, generally, polymerization will be conducted at somewhat lower temperatures, which preferably will not fall below a temperature of 150xc2x0 C.
The polymerization may of course also be conducted in the presence of polymerization regulators (chain transfer agents). These include compounds containing SH groups, and halogenated hydrocarbons such as chloroform, bromoform or carbon tetrachloride. Furthermore, alcohols which may be used as solvents in the process of the invention may act to a smaller extent as chain regulators. Preferably, the polymerization process of the invention is conducted in the absence of chain regulators, apart from the aforementioned alcohols.
Solvents are used, where intended, in amounts of up to 50% by weight, in particular up to 30% by weight, based on the overall weight of the monomers. Examples of suitable solvents are alcohols, e.g. n-propanol, isopropanol, n-butanol, isobutanol, ether alcohols such as ethyl glycol ether, ethyl diglycol ether, ethers such as ethylene glcyol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, esters such as butyl acetate, ketones such as methyl amyl ketone, aromatic alcohols such as benzyl alcohol, methylbenzyl alcohol, cumene alcohols, and also aromatic and aliphatic hydrocarbons such as toluene, xylene, ethylbenzene, cumene, diisopropylbenzene, diphenylmethane, tetralin or decalin.
The process of the invention is generally conducted under superatmospheric pressure, because the starting materials are volatile at the reaction temperatures. In practice, the reaction is carried out under the autogenous vapor pressure of the polymerization mixture at reaction temperature.
The polymerization may be conducted continuously or discontinuously in the reaction vessels appropriate for the purpose. Appropriate polymerization vessels for the discontinuous conduct of the polymerization are customary stirred tank reactors which are preferably designed for working under superatmospheric pressure.
The discontinuous implementation of the process of the invention may be configured as a batch, semibatch or feed process. In the case of the batch process, the monomers to be polymerized, the polymerization initiator and any solvent envisaged are introduced into the polymerization reactor and heated to polymerization temperature, this temperature being maintained until the desired conversion has been reached. In the case of the semibatch process, some of the polymerization initiator and any solvent desired is charged to the reactor in the monomers intended for polymerization and this mixture is heated to polymerization temperature, after which the remaining amount of initiator is added in the course of the polymerization.
In the case of the feed process, solvent and, if desired, a portion of the monomers to be polymerized and, if desired, a portion of the polymerization initiator are charged to the polymerization vessel, but preferably do not represent more than 20% of the monomers to be polymerized and of the initiator required; the mixture is heated to polymerization temperature, and then the remaining amounts of polymerization initiator and the monomers to be polymerized are added under polymerization conditions. The rate of addition of the monomers to be polymerized and of the initiator is guided of course by the polymerization rate, the heat of polymerization which is released, and the dissipation of heat from the polymerization vessel. The feed period is generally in the range from 10 minutes to 4 hours and preferably in the range from 30 minutes to 2 hours. It is commonly followed by a postpolymerization phase for the purpose of completing the conversion. This may last several minutes, e.g., from 10 to 20 minutes, up to 5 h, and will preferably not exceed 2 h.
Besides stirred tank reactors, suitable polymerization vessels for the continuous implementation of the process of the invention include tube reactors, loop reactors, annular gap reactors, preferably those having an annular gap with a width of from 0.5 to 10 mm, as described for example in DE 42 03 277 and DE 42 03 278. Reference is hereby made to those documents. Annular gap reactors are preferred for the implementation of the process of the invention. Examples of suitable annular gap reactors also include those where the conditions for Taylor vortex flow are met. Such reactors are described, for example, in DE-A 198 28 742, which is likewise fully incorporated herein by reference.
In order to implement the continuous configuration of the process of the invention, the starting materials will generally be fed to the polymerization reactor via separate feed streams or, preferably, as a mixture of initiator, monomers, and where appropriate, solvent, and the resulting polymer will be removed continuously. Depending on the reactivity of the monomers to be polymerized, the desired degree of conversion, and the reactor geometry, the average residence time of the reaction mixture to be polymerized in the reactor is from 1 minute up to 2 hours and in particular is in the range from 2 minutes to 0.5 hour.
In this context it has proven appropriate to recycle part of the product stream removed from the reactor back into the polymerization reactor together with fresh monomers and initiator. The ratio of recycled product stream to the amount of the freshly supplied monomers+initiator is also referred to as the return ratio and is preferably in the range from 5 to 30.
Subsequent to the polymerization it is common to provide for removal of the unreacted monomers, any volatile byproducts, any initiator decomposition products present, and the solvents used where appropriate. Suitably for this purpose it is possible to employ one or more thin-film evaporators which are operated continuously and are preferably connected in series. The monomers separated off, and any solvents, may of course be returned to the process. Where appropriate, byproducts such as initiator decomposition products and unwanted oligomers may be separated off beforehand in a separate process step.
The polymers obtained by the process of the invention, unlike crosslinked polymers, are soluble in a range of organic solvents, which is an indicator of their low molecular weight. Suitable solvents for the polymers of the invention are, for example, ketones such as acetone, methyl ethyl ketone and cyclohexanone, aromatic hydrocarbons such as xylenes and toluene or mixtures thereof, alcohols such as ethanol, n-propanol, isopropanol, n-butanol and isobutanol, ether alcohols, e.g. ethylene glycol monomethyl ether, or relatively high-boiling petroleum fractions having boiling points  greater than 140xc2x0 C., and also mixtures of the aforementioned solvents.
The branched polymers of the invention differ from the polymers of comparable molecular weight known to date in the presence of defined branching sites. The fraction of branching sites depends on the amount of the monomers B used and is generally in the range from 0.01 mol to 1 mol and preferably in the range from 0.05 mol to 0.5 mol/kg.
The partially branched polymers prepared by the process exhibit improved weathering stability, for a given molecular weight, relative to the linear polymers of similar monomer compositions.
As a result of the fact that the branching of the chains was formed right in the low molecular weight structure, it is possible using the appropriate cocrosslinkers to use them to synthesize macropolymers having very high flexibility. This high flexibility is important, for example, in connection with the coating of glass fiber cables (primary coating, secondary coating) by means of UV-sensitive coating materials, or with the preparation of polyurethane foams.
Depending on the nature of the copolymerized monomers A, the polymers of the invention are suitable for example as binders for coatings such as paints, protective coatings, printing inks, adhesives or paper coating compositions, and also as building blocks in thermosettable coating systems and in polyesters and polyurethanes. The hydroxy-functionalized polymers of the invention are used in particular in the preparation of polyurethanes and with particular preference in the preparation of polyurethane foams based on aliphatic isocyanates.